[petsc-users] Optimized mode

[Emmanuel Ayala]

Hi, thanks for the quick reply.


El lun., 16 de sep. de 2019 a la(s) 23:40, Smith, Barry F. (
bsmith at mcs.anl.gov) escribió:

>
>   What do you mean by 2.0e-4 units ?   If you mean the last 4 digits may
> differ in the two solutions,


yes, that is the meaning

yes that is completely normal. How many digits you lose depends on the
> order of the operations  and the condition number of the matrix and and for
> elasticity that will very easily be  greater than 10^4
>

OK. I understand that two solution can produce slightly different results.
So,  I guess, the optimized mode produce slightly different results because
internally something change (order of some operations, maybe) when the
number of process change?.

>
>   Barry
>
> From:
> https://pdfs.semanticscholar.org/dccf/d6daa35fc9d585de1f927c58cc29c4cd0bab.pdf
>
> We conclude this section by noting the need for care in interpreting the
> forward error. Experiments in [24] show that simply changing the order of
> evaluation of an inner product in the substitution algorithm for solution
> of a triangular system can change the forward error in the computed
> solution by orders of magnitude. This means, for example, that it is
> dangerous to compare different codes or algorithms solely in terms of
> observed forward errors.
>
> Thank you very much!
Best regards.

>
> > On Sep 16, 2019, at 11:13 PM, Emmanuel Ayala via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
> >
> > Hi everyone,
> >
> > I have a code for elastic linear analysis which involves DMDA, SuperLu
> and matrix operations. I have a MPI vector which contains the nodal
> displacements, when I run the code in debug mode i get the same value for
> the norm (2 or Inf), for different number of process. But when I use the
> optimized mode, I have small variations for the same norm depending on the
> number of process.
> >
> > -Debug mode: the same norm value for any number of processes.
> > -Optimized mode: the norm value changes with the number of processes.
> The variations are around 2.0e-4 units.
> >
> >  This is normal?
> >
> > For my optimized mode I used the next configuration
> >
> > ./configure --with-debugging=0 COPTFLAGS='-O3 -march=native
> -mtune=native' CXXOPTFLAGS='-O3 -march=native -mtune=native' FOPTFLAGS='-O3
> -march=native -mtune=native' --download-mpich --download-superlu_dist
> --download-metis --download-parmetis --download-cmake
> --download-fblaslapack=1 --with-cxx-dialect=C++11
> >
> > Best regards.
>
>
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